In order for a mechanical seal as an example of a sliding component, to maintain sealing performance for a long period of time, the sliding component needs to satisfy both conflicting conditions of “sealing” and “lubrication.” In particular, in recent years, for environmental measures or the like, there has been an increasing demand for a further friction reduction to reduce mechanical loss while preventing leakage of a sealed fluid. A technique to reduce friction can be achieved by generating dynamic pressure between sliding faces by rotation to bring the sliding faces into a so-called fluid lubrication state where the sliding faces slide with a liquid film interposed therebetween. However, in this case, positive pressure is generated between the sliding faces; therefore, fluid flows out from a positive-pressure portion to the outside of the sliding faces. That is a side leakage in bearings, which corresponds to a leakage in seals.
To achieve both sealing and lubrication, the present applicant has previously filed an application for an invention (hereinafter, referred to as a “prior art”, see Patent Document 1) in which, as shown in FIG. 8, dimples 50 are provided in one of sliding faces S of a pair of sliding parts that slide relative to each other, and an upstream cavitation formation region 50a of each of the dimples 50 is disposed close to the low-pressure fluid side and a downstream positive pressure generation region 50b of each of the dimples 50 is disposed close to the high-pressure fluid side. A fluid drawn into the cavitation formation region 50a of each dimple 50 passes through the dimple 50 and is returned from the positive pressure generation region 50b toward the high-pressure fluid side and therefore leakage is reduced. At the same time, a liquid film is held between the sliding faces by positive pressure generated in the positive pressure generation region 50b and therefore wear is reduced.
The prior art described in Patent Document 1 is a breakthrough invention that achieves both sealing and lubrication.
The invention of the present application relates to improvements in the prior art.
It has been found by experiments that in the prior art described in Patent Document 1, depending on the type, temperature, and pressure of the sealed fluid, the sliding speed of the sliding face, and the like, the cavitation formation region 50a does not extend toward the downstream side and the positive pressure generation region 50b is excessively increased and therefore sealing performance is reduced.
For example, when the sliding speed of the sliding face is changed under the condition where the type, temperature, and pressure of the sealed fluid are fixed, a phenomenon where leakage is found at middle rpms and leakage is hardly found at high rpms has been observed.